The present invention relates to non-aqueous electrolyte secondary batteries, particularly to non-aqueous electrolyte secondary batteries (hereinafter referred to as xe2x80x9csecondary batteriesxe2x80x9d) with high energy density and improved electrochemical characteristics, such as charge/discharge capacity and cycle life, that are provided by the improved negative electrode material and non-aqueous electrolyte.
High electromotive force and high energy density are featured by lithium secondary batteries recently used for such mobile communications equipment as personal digital assistants and mobile electronic apparatus , main power supply for the mobile electronic gears, small domestic power storage devices, and motor bicycles, electric vehicles, and hybrid cars using motors as their driving sources.
Lithium ion secondary batteries using organic electrolytic solutions, carbon materials as their negative electrode active materials, and lithium-containing composite oxides as their positive electrode active materials have higher energy density and more excellent low-temperature characteristics than secondary batteries using aqueous solutions. Moreover, without using lithium metals for their negative electrodes, lithium ion secondary batteries also have excellent cycle stability and safety, thus rapidly becoming commercially practical. Lithium polymer batteries using electrolytes like macromolecular (polymer) gel containing organic electrolytic solutions are also being developed as a new battery family of thin and light type.
When a lithium metal with high capacity is used as negative electrode material, dendrite-like deposition is formed on the negative electrode during charging. During repeated charge and discharge operations, the dendrite may penetrate the separator or polymer gel electrolyte, reach the positive electrode side and thus cause internal short circuits. Having a large specific surface area and thus high reactivity, the lithium deposition reacts with the solvent, becomes inactive, and decreases the charge/discharge efficiency. This phenomenon increases the internal resistance of the battery and produces particles isolated from electronic conduction network, thus decreasing the charge/discharge efficiency of the battery. For these reasons, lithium secondary batteries using lithium metals as their negative electrode materials have problems of having low reliability and poor cycle life characteristics.
At present, lithium secondary batteries use, for their negative electrodes, carbon materials capable of intercalating and de-intercalating lithium ions, and have become commercially available. Generally, since metal lithium does not deposit on a carbon negative electrode, it does not cause the problem of internal short circuits resulting from the production of dendrite. However, the theoretical capacity of graphite, one of carbon materials now in use, is 372 mAh/g, which is so small as one-tenth the theoretical capacity of pure Li metal.
As other negative electrode materials, metallic and non-metallic pure elements that form compounds with lithium are known. For example, the composition formula of a compound of tin (Sn), silicon (Si), or zinc (Zn) containing largest amount of lithium is expressed by Li22Sn5, Li22Si5, or LiZn, respectively. Within this composition range, generally, no metallic lithium deposits; therefore, there is no problem of internal short circuits caused by the formation of dendrite. The electrochemical capacities between these compounds and each of the pure elements are 993 mAh/g, 4,199 mAh/g, and 410 mAh/g, respectively, all of which are larger than that of graphite.
Other compounds for negative electrode materials proposed include a nonferrous metal silicide consisting of transition elements disclosed in Japanese Patent Laid-Open Publication No. H07-240201, and a negative electrode material consisting of an inter-metallic compound containing at least one element selected from Group 4B elements, P, and Sb and having a crystal structure of one of CaF2, ZnS, or AlLiSi type disclosed in Japanese Patent Laid-Open Publication No.H09-63651.
However, each of the aforementioned negative electrode materials with high capacity has the following problems.
Generally, the negative electrode materials of metallic and non-metallic pure elements that form compounds with lithium exhibit poorer charge/discharge cycle characteristics than carbon negative electrode materials. This is probably because the negative electrode materials break by their volumetric expansion and shrinkage.
Meanwhile, unlike the aforementioned pure elements, a negative electrode material that consists of a nonferrous metal silicide consisting of transition elements and an inter-metallic compound that contains at least one element selected from Group 4B elements, P, and Sb and has a crystal structure of one of CaF2, ZnS, or AlLiSi type are proposed as negative electrode materials with improved cycle life characteristics in Japanese Patent Laid-Open Publication No.H07-240201 and No.H09-63651, respectively. In Japanese Patent laid-Open Publication No.H10-208741, the range of nuclear magnetic resonance (hereinafter abbreviated as NMR) signals of lithium intercalated in the negative electrode material is proposed.
In the battery using the nonferrous metal silicide consisting of transition elements as the negative electrode material disclosed in Japanese Patent Laid-Open Publication No.H07-240201, while the discharge capacities at the first, fiftieth and hundredth cycles shown in its examples and a comparative example indicate that its charge/discharge cycle characteristics have been more improved than those of the lithium metal negative electrode material , the discharge capacities have only increased by about 12% at maximum compared with that of the natural graphite negative electrode material.
For the material disclosed in Japanese Patent Laid-Open Publication No.H09-63651, its examples and comparative examples show that it has more improved charge/discharge cycle characteristics than the negative electrode material of Lixe2x80x94Pb alloy and has higher discharge capacity than graphite. However, the battery considerably decreases its discharge capacity at 10 to 20 charge/discharge cycles and even with Mg2Sn that is considered best, its capacity decreases to 70% of its initial capacity after about 20 cycles. For this reason, this material is inferior in charge/discharge characteristics.
For the material disclosed in Japanese Patent Laid-Open Publication No. H10-208741, NMR signals of the lithium intercalated in various alloys appear within the range of 5 to 40 ppm. By using this material, an electrode material with high energy density and excellent cycle life is proposed. However, as for the cycle life of the battery using this electrode material, its capacity decreases to 70% of its maximum after 372 cycles even when LiCoO2 is used for its positive electrode.
The present invention addresses the aforementioned problems conventional batteries have.
The negative electrode of the battery of the present invention is characterized by the use of composite particles. In the composite particles, nucleus particles containing at least one element selected from tin , silicon, and zinc as their primary constituent element are covered with a solid solution or inter-metallic compound of the elements constituting the nucleus particles and at least one element selected from groups consisting of Group 2 elements, transition elements, and Group 12, Group 13, and Group 14 elements in the Periodic Table except for carbon and constituent element of the nucleus particles. Further, the negative electrode of the battery of the present invention is characterized in that NMR signals of the lithium intercalated in the negative electrode material appear within the range of xe2x88x9210 to 40 ppm with respect to lithium chloride as a reference and at least one NMR signal appears within the range of xe2x88x9210 to 4 ppm. In addition , the negative electrode of the battery of the present invention is characterized in that the above NMR signals with respect to lithium chloride as a reference appear within the range of xe2x88x9210 to 4 ppm and xe2x88x9210 to 20 ppm and the NMR signal intensity appearing within the range of xe2x88x9210 to 4 ppm is 1 to 10 times as large as those appearing within the range of 10 to 20 ppm.
With the aforementioned structure, non-aqueous electrolyte secondary batteries that address the problems of conventional batteries and have higher energy density and more excellent cycle characteristics can be provided.